#3311 docs: improve discoverability and docs for the instructions parameter

Summary

Improve discoverability and documentation of the instructions parameter on
MCPServer (which flows through to InitializeResult.instructions).

Closes #1464

Changes

• Added a docstring to the instructions parameter in MCPServer.__init__,
  explaining its purpose and hinting at tool-grouping / workflow use cases.
• Added a new "Server instructions" section to docs/get-started/first-steps.md
  with a concrete example showing how to use instructions to express tool
  grouping and ordering guidance.
• Added docs_src/first_steps/tutorial002.py as the referenced example.

Checklist

• Linked issue: #1464 (documentation, ready for work, P1)
• Change is minimal and focused
• Docs example follows the existing --8<-- include pattern
• No code logic changes (docs + docstring only)

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🤖 AI-assisted: This PR was drafted with AI assistance. I (the contributor)
have reviewed the diff, understand the change, and confirm the example
accurately demonstrates the instructions parameter.